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Almost Earth-like Exoplanets

NASA’s Kepler mission has discovered a 5-planet system that includes a hot Mars and four super-Earths, two of which might host liquid water. These aren’t quite the Earth-like exoplanets Kepler’s been looking for, but they’re close.

An artist imagines what the Kepler-62 system might look like. The small shining object seen to the right of Kepler-62f is Kepler-62e. Orbiting on the inner edge of the habitable zone, Kepler-62e is roughly 60 percent larger than Earth. The continents and oceans shown here exist only in the artist's imagination.

NASA Ames / JPL-Caltech

In a system 1,200 light-years away, five small planets whizz around an orange star about two-thirds the size of our Sun. One of them is a hot Mars, a planet half the size of Earth that whips around the star every 12.4 days. Another two are hot super-Earths, completing orbits every 5.7 days and 18.2 days. But the final two are the ones that capture the imagination: half again the size of Earth and orbiting in the star’s habitable zone, these worlds could be the pale blue dots we’ve been looking for. Maybe.

“I'd be hesitant to call any of these worlds potentially 'Earth-like.'” says Caleb Scharf (Columbia University), an exoplanet expert not involved in the study. “But their discovery is definitely leading us closer and closer to places that might represent alien, but nonetheless similar, environments to our own.”

The planets Kepler-62e and Kepler-62f are 60% and 40% bigger than Earth, respectively, and their orbits last 122 days and 267 days, William Borucki (NASA Ames Research Center) and his team report this week in the journal Science. But the team is careful to point out that planets so small are difficult to characterize — even pinning down their masses will be a challenge.

To find these planets, Kepler has been snapping pictures of more than 150,000 stars roughly every 30 minutes for more than four years now, looking for the periodic dips in brightness that could signal a planet passing in front of its star. Kepler-62 is one of these, a hydrogen-fusing K2 star almost twice as old as the Sun.

This diagram compares the planets of the inner solar system to those orbiting Kepler-62.

NASA Ames / JPL-Caltech

Kepler’s automatic pipeline searches the hundreds of thousands of stars for brightness variations, and this pipeline discovered four of the five exoplanets. But the fifth, Kepler-62f, required a more hands-on approach. The planet has only made three transits so far, and one of these transits came too close to a data-transfer break for the pipeline’s liking. A more careful look by coauthors Eric Agol and Brian Lee (University of Washington) identified the third transit, upgrading the signal to planet status.

The holy grail of exoplanet searches is finding an Earth-sized planet in a star’s habitable zone, but that’s more easily defined in words — “the region around a star where a rocky planet with an atmosphere could host liquid water on its surface” — than in practice. Borucki’s team used two approaches to measure the habitable zone. The first assumes any planets are rocky with carbon dioxide-water vapor atmospheres. Under these strict assumptions, Kepler-62f would receive enough stellar flux to keep liquid water on its surface with the help of the greenhouse effects, but Kepler-62e would be too hot.

A more liberal approach defines the habitable zone by calculating the solar flux Venus and Mars received when they were still able to host liquid water. This definition allows Kepler-62e to sneak into the warm end of the habitable zone.

Even if we take the latter tack, are the planets actually habitable? Their masses are too low to nail down via wiggles in the star’s position or variations in the transits’ timing, so with just their sizes we can only speculate about what they’re made of. Still, by not finding these other signals, the astronomers can limit Kepler-62e to 36 Earth masses and Kepler-62f to 35 Earth masses. Those limits are enough to confirm that the bodies are planets.

The two planets’ diameters, combined with their distance from the star and the star’s age, suggest both worlds have likely lost any thick, Jupiter-esque gas envelope that might have enshrouded them in their early years. So, depending on their exact masses, the planets are either waterworlds or rocky.

This illustration shows the relative sizes of Kepler-discovered super-Earths (left to right): Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f. The rightmost planet is of course Earth, the only planet not shown as an artist's rendition.

NASA Ames / JPL-Caltech.

“If they're Earth-mass, they'll be strange water-rich worlds,” Scharf speculates. “It's not inconceivable that they could harbor a liquid water ocean close to the surface, but this would presumably have to be a very deep ocean, turning to increasingly pressurized phases of ice at depth.”

Even if the planets are many times Earth’s mass, they could be still rocky instead of watery, Scharf says. But at such large masses, the rocky planets would be decidedly different from Earth, possibly very geologically active. Then again, since Kepler-62 is 7 billion years old, there’s also a chance the planets could have simmered down by now.

There’s still a chance of getting a better answer someday. By continuing to measure transits over many years, the team could one day determine exact masses by measuring the gravitational effect of interplanetary tugs. With Kepler scheduled to fly at least until 2016 and possibly beyond, there’s a chance this method could one day provide better answers about these planets’ nature.

13 thoughts on “Almost Earth-like Exoplanets”

Nice article and excellent news. Thanks for the informative links. The one from NASA includes a video that shouldn’t be missed. Ok, so now is when the Kepler news starts getting really, really interesting as the harder to find earthlike planets in earthlike orbits around sun-like stars begin to emerge from the data. Of the three discussed here Kepler-62f seems the most earthlike and interesting. The key facts are that it is 40% larger than earth and that it receives (from the UofW link) “about half as much solar flux” as Earth. At first this may sound like it would be an ice world, but with its larger mass wouldn’t it also be likely for it to have a thicker, more heat retaining atmosphere?

Bruce, enjoyed your comments. I will slow enthusiasm down a bit here. We need to consider HR 8799 and reports now on the atmospheres measured recently. The exoplanet atmospheres have model problems in this system, different than expected. Also remember reports of Kepler-10b exoplanet some 40% larger than earth. It is as dense as a cannonball reported back in April 2011 issue of S&T. I would be careful about habitable earth claims for exoplanets 40-60% larger than earth. The report here is careful to reveal that the potential habitable zone exoplanets could be as large as 35-36 earth masses. Such bodies in the heavens can be quite different than our earth.

Mike, as far as I can tell Kepler-62 and Kepler-69 are consistently reported with estimated distances of 1,200 and 2,700 light years respectively. The distances to these two stars aren’t the key thing though. (It’s not like we’re going to be able to go to them, as far as anyone knows.) The key thing with all these Kepler as well as other exoplanet discoveries is their representative nature, because for each find there might be thousands or millions of other star systems with similar characteristics in the Milky Way alone, only they are undetectable due to having the wrong orbital angle, small planet size, length of period, attenuation of signal due to distance, not being in the field Kepler or another telescope is looking at, etc., etc. Rod, I enjoy reading your comments as well, and I acknowledge your words of caution about these Super Earth finds. Indeed, even if they bag a perfect Earth – Sun analog there could still be a host of reasons why it might be unsuitable for life. (Wrong chemistry, spin axis, rotation rate, eccentric orbit, frequent massive impactors, etc.) And the number one reason there might not be life on any given planet: no one put it there. (Life being far too complex for spontaneous generation.) Still, I like planets, and the more the merrier.

Well, MY NASA video clearly says Kepler-62 is 360 light years away, plus, yes, Kepler-69 is 2’700. Something must be broken in the chain (and including the end points!) between my brains and the NASA site. — Life on planets: I often wondered what we all would accept/ expect to be called "life" on other planets. It is even hard to clearly define that term on our own planet. Let alone, given its adaptive powers, would we safely be able to discover "life" elsewhere, even if right in front of our eyes?

But on the other hand Rod, I still think this Kepler-62f is potentially one of the most interesting planets the mission has uncovered to date. The metallicity of Kepler 62 is listed as -.37 dex, which I think (if I worked the logarithms right, please correct me if I didn’t) means that this system started out with about 57% of the metals that our system began with, which makes sense if this system is around 7 BY old. With fewer metals to begin with I would think that the 34-36 mass limits would constitute rather unlikely possibilities. These masses are merely maximum limits based on very limited or lacking transit timing variations, and the Kepler database lists 62f’s mass as <34 earths. I built a simple spreadsheet to estimate various characteristics of exoplanets, using the known radius and estimated density as inputs. With a radius 41% larger than earth, 62f has a surface area almost twice that of Earth’s. But even if Kepler-62f has an average density 50% larger than Earth, with a radius of 1.41 earths its mass would still be only 4.2 earths. If this exoplanet has less iron and other heavy elements by percentage while having a very deep global ocean it would have to be LESS dense than the Earth, would it not? If my calculations are correct an average density of 71% of Earth’s would give it a mass of only 2 earths and a surface gravity equal to ours! If you were there and had a very sea-worthy vessel you might be able sail its vast equatorial ocean, though you might need to dodge icebergs. If it had plenty of fish to eat penguins might find it quite appealing.

Bruce, thank you for your comment about no one putting life there. I most heartily agree with that sentiment. If you take life there as a given, though, the more densely packed the foliage, the lower the planet’s albedo; the lower the planet’s albedo, the warmer the planet. So, if it has a global rain forest, the climate might be more like oregon than Alaska. Conversely, on 62e, a highly desertified world would mean a high albedo; a high albedo, of course, lowers the planet’s temperature. I know it seems counter-intuitive to some, but with a biosphere that would seem to be cooler, the planet warms up, and with one that would seem to be hotter, the planet cools down. This is because the surface of the "cooler" biosphere is a darker surface and the surface of a desert is a much brighter surface.

Billsey, thank you for your thought provoking comment, and not just for our agreement as to life needing an Originator. Your points about albedo changes due to the presence or absence of plant cover on a planet’s surface are sound, demonstrating a moderating effect on the extremes that a life bearing planet can have as opposed to a lifeless one. You could say that even mindless plants can work to make their environments less hostile. Animal life sometimes actively changes the environment, humans being the ultimate example. Your phrase, “If you take life as a given” is very interesting though. Can we do that, at this point? Billsey, was your expression just a figure of speech, or a bold prediction? I don’t think so. As yet we still have an example of only one life bearing planet out of the uncountable total that must be out there. The fledgling science of astrobiology has made bold extrapolations (from the example of one), reasoning that since life evolved here, it must have also evolved elsewhere, perhaps even commonly. Taking a page from their playbook, those of us who believe in creation could reason, since life was created here, God could have created it elsewhere, and since here on Earth every conceivable habitat for life is inhabited by organisms of one form or many, exoplanets in favorable zones might be full of life also, placed there by the One the Bible calls “the source of life.” (Psalms 36:9) I’m not making a prediction though, just considering possibilities.

Bruce about your speculations. I checked out the URL, http://exoplanet.eu/. There are different reports available and the transiting exoplanets published masses and radii. Working from that source I found super-earths where the masses are <=10 earth masses, some < 1 earth mass. The average mean density for the super-earths is quite large, 16.45 g cm^-3. Quite a few of these super-earths mean density is > 6 g cm^-3. The real question Bruce, why would Kepler-62 f exist so far off the trend for the super-earths already documented by the transit method? The existing body of super-earths documented with well determined masses and radii suggest these bodies have large mean densities as a general rule.

Rod, interesting question. First off for comparison I looked up the densities of the rocky planets in our system: Mercury 5.427 g/cm^3, Venus 5.243, Earth 5.515, and Mars 3.9335, so the average in our system is a little over 5 g/cm^3. (The densities of the giant planets are; Jupiter 1.326, Saturn 0.687, Uranus 1.27 and Neptune 1.638 g/cm^3.) As you note Rod to date the exoplanet finds include many very dense super-earths, and to be so dense it would be logical to assume that they would be rocky, and generally the more massive the denser as their cores and mantles experience greater gravitational compression. But most or maybe even all of these super dense exoplanets are also in the hot, close to their stars class. Could some of these be the remaining rocky cores of once giant planets that migrated in and then had their gaseous envelopes stripped away by stellar winds? Whatever the case, it should be remembered that solar type planetary systems are still the exception, not the rule. The close in heavyweights are much easier to find and to characterize than smaller planets orbiting way out in the more temperate zones. As for Kepler 62f, it’s on a 267 day orbit around a cooler than the sun Orange Dwarf. The frost line of this system would therefore have been closer in than ours was, so 62f could have received a good supply of H2O to begin with and with its large mass it would have been able to retain it (unlike Mars). Also there is the lower iron content in the star Kepler 62 to factor in. If there were less of the heaver elements in the system at the start I would expect that there would be lower percentages of the Fe to U elements in 62f’s core as compared to Earth’s. Therefore I think that an earthlike or even a sub-earth density (if it’s a deep ocean water world) might be possible for this world. The possibility of penguiniformic life would be practically nil however. 

Bruce, you wax eloquently in your speculations. A careful study of the URL, http://kepler.nasa.gov/Mission/discoveries/ has some interesting items posted. Kepler-62 f is 208K surface temperature with e = 0.15, a fairly large eccentricity for its orbit which is more like Mercury in our solar system and 208K is very cold. Mean density < 85 g cm^-3. Kepler-62 e is listed as 270K with e = 0.10, more like Mars eccentricity in our solar system and mean density < 47 g cm^-3. Both of these exoplanets have greater eccentricity than earth which is e = 0.0167. The host star metallicity is Fe/H = -0.37 which is 43% solar value. These exoplanets do not look very earthlike in my opinion and we lack firm measurements for mass and diameters to compute accurate mean densities too. So both bodies could just be gaseous exoplanets as well or dense cannonballs. Two exoplanets in the table serve as examples about speculations. Kepler-10 b mean density is 8.8 g cm^-3, surface temperature 1833K and its host star metallicity, Fe/H = -0.15 or 71% solar value yet it is as dense as an iron cannonball. Kepler-11 f is 2.3 earth masses with mean density 0.7 g cm^-3 or about Saturn’s mean density and surface temperature 544K. Its semi-major axis is 0.25 AU or just closer than Mercury and its host star metallicity is 0 or equal to the solar value. Speculations need to be replaced with good science

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